Automotive radars using millimeter waves are attracting interest as being most suitable for use in the prevention of car collisions and in following another vehicle as they are less susceptible to the interference of rain, fog, snow and other weather factors, dust and noise than ultrasonic radars and laser radars. Millimeter wave radars currently available in the market are mainly intended for use along expressways, and their detectable range and reach are approximately 16 degrees in azimuth angle and about 150 m in distance, respectively.
Automotive radars are further expected to be applied in the near future to a more complex road environment, such as ordinary roads, and this would inevitably necessitate a broader detectable range of 80 degrees or more in azimuth angle.
Techniques available to meet this need include a method widening a receiving angle of antenna on the basis of use of a plurality of receiving antennas and a monopulse system of detecting the azimuth of the obstacle from the amplitude difference or the phase difference among the signals received by the receiving antennas. For instance, non-patent reference 1: 2001 General Conference of the Institute of Electronics, Information and Communication Engineers, Technical Papers (Engineering Science), Paper No. A-17-10, p. 391 discloses a technique to broaden the detectable range by using a monopulse system for azimuth detection and keeping the number of antenna elements intentionally small. Besides that, for instance, non-patent reference 2: MWE 2001 (Microwave Workshops and Exhibition) held in Yokohama in December, 2001, Workshop Technical Program, Paper No. WS5-1 contains a description of an automotive radar transmitter/receiver apparatus embodied in such a method.
FIG. 11 shows the circuit configuration of a radar transmitter/receiver apparatus described in the non-patent reference 2. For a wide-angle transmitting antenna 1 and receiving antennas 2a and 2b, microstrip-patch planar antennas based on microstrip lines are used. A millimeter wave signal outputted by an oscillator 7 is supplied to the transmitting antenna 1 via a power amplifier 6. The signal transmitted from the transmitting antenna 1 and reflected by an obstacle is received by the receiving antennas 2a and 2b, and supplied to a hybrid circuit (HYB) 5. The hybrid circuit 5 generates a sum signal Σ and a differential signal Δ. The sum signal Σ and the differential signal Δ are respectively processed by receiver circuits 20a and 20b, each mainly consisting of a mixer to detect the azimuth and other factors of the object of detection.